Patent Document 1 (JP-A No. 2006-506648) discloses a flow cell related to a flow cytometer for analyzing particles in a sample fluid, in which a sample is caused to flow in the center of a sheath fluid using a cannula. In this flow cell, the sheath fluid and the sample fluid are merged and flow together, the fluid as a whole is pressed and flattened by a narrow flow channel at the downstream of the flow cell, and the sample fluid is imaged with a camera.
In the configuration disclosed in Patent Document 1 (JP-A No. 2006-506648), the sample fluid and the sheath fluid mix together at a flow merging point. After that, even if fluid is constricted, there is a possibility that part of the sample fluid lies outside the focal range of the optical system, which is referred to as the photographic depth of field. Moreover, in this configuration, a pump system is required in order to control fluids at high precision, with the device itself becoming complicated and costly. Moreover, due to there being a lot of sheath fluid between the sample and the camera, the optical distortion due to flow in the sheath fluid and the like is also captured at the same time, leading to a detrimental effect on the captured images. Moreover, due to the large volume of the flow cell, a lot of sheath fluid is consumed, and the structure of the flow cell is complicated, raising production costs.